1. Technical field of the invention
The present invention concerns a hydraulic damper designed to be fitted between the chassis or the bodywork of an automobile vehicle and a non-suspended part such as a road wheel of the vehicle, for example.
2. Description of the prior art
The invention relates in particular to dampers comprising a control valve adapted to reduce the damping force from a particular rate of inward (compression) movement of the damper piston rod. Dampers of this type are already known, being described for example in French patent publication No. 2687627. (SIRVEN); to this end they comprise a control valve responsive to the hydraulic fluid pressure and which, when it opens, establishes communication between the two chambers delimited in the damper cylinder by the main piston driven by the damper piston rod. The pressure controlling the control valve is obtained by retarding the flow towards a reservoir of excess fluid due to the reduction in the active volume of the cylinder as a result of the penetration of the piston rod into one of the chambers of the cylinder. If during a rapid inward movement the speed of the piston rod exceeds a limit value the differential pressure acting on the two sides of the control valve increases and, beyond a predetermined limit value, opens the control valve which leads to a rapid reduction in the damping force. There is therefore obtained in this way a damping force which decreases with the rate of inward movement of the damper piston.
The control valve may also be acted on by a substantially constant reference pressure, one side of the control valve then possibly constituting a part of a surface delimiting a reference chamber filled with a reference fluid, for example a gas at a predetermined pressure. This reference chamber may be closed and isolated or vented to the atmosphere or connected to the part of the fluid reservoir filled with gas in such a way as to achieve pressure equilibrium.
European patent publication 3290 (SIRVEN) describes a damper of this type in which the control valve is attached to the piston, instead of being fixed in the cylinder. The hydraulic fluid reservoir is advantageously disposed in the upper part of a tube defining the cylinder, at the opposite end to the piston rod.
In both these known type dampers there is further provided inside the damper a resilient block, made from an elastomer material, for example, capable of varying the active volume of the damper during rapid inward movement of the piston rod and so modifying the operating characteristics during a sudden compression by opening the control valve to increase the pressure inside the damper.
In these known dampers the flow of hydraulic fluid from the reservoir to the cylinder during outward (expansion) movement of the piston rod is virtually unrestricted through a passage connecting the reservoir to the damper cylinder, this passage being provided with a check valve enabling flow only from the reservoir to the cylinder.
There are already known devices for varying the operating characteristics of vehicle dampers of conventional type, whether of single-tube or double-tube construction. These variation devices generally employ solenoid valves opening or closing off oil passages or electric motors adapted to move plungers in flow restrictors through which the oil in the damper passes. These passages or these restrictors create the pressure drop which is at the origin of the damping force. During a compression movement, the damping force is increased because the passage is smaller, this change in operation applying to all rates of compression.
To modify the characteristics of a conventional damper in compression the restrictor control means are in the piston in the case of a single-tube damper which results in difficulties in locating the control means and operating them from the outside. In the case of a double-tube damper, that is to say when the damper cylinder is mounted inside an outer cylindrical jacket which comprises the reservoir, the control means for the restrictors designed to modify the damper characteristics in compression may be disposed either in the piston or in the vicinity of a valve situated at the end of the cylinder.
To modify the characteristics of conventional dampers during an expansion movement consideration has also been given to providing control means to vary the flow rate through flow restrictors provided in the piston itself which results in the previously mentioned difficulties with locating and operating the control means.
An object of the present invention is to modify dampers of the type mentioned in the preamble hereto so as to enable their operating characteristics to be varied at will, and in particular allowing for various parameters related to movement of the vehicle.